1. Field of the Invention
This invention relates generally to a process for removing contaminants from a gas stream.
More particularly, this invention relates to the removal of volatile organic compounds from air streams such as those produced in the treatment of wastes.
The build-up of volatile organic compounds in ambient air is of increasing concern as such compounds are now recognized as a major source of air pollution in many urban areas. Many volatile organic compounds are released into the air through the inevitable leaks and spills accompanying industrial processes and chemical manufacture and these sources of pollution have had much attention. It has recently become evident that the treatment of municipal liquid wastes also releases substantial quantities of volatile organic compounds into the atmosphere. Some of these compounds find their way into collection systems as oils and other wastes which are dumped into sewers and some by run-off from rain washing streets with residues from automotive traffic. No satisfactory means for control or capture of these pollutants is presently available.
Municipal waste water treatment facilities liberate volatile organic compounds in a number of different treatment operations. Such operations include, for example, pumping stations, trickling filters, aerobic digesters, aeration basins and the like. The variety and quantity of volatile organic compounds liberated into the atmosphere in such unit operations include many compounds that one would not ordinarily expect from sewage sources. Major classes of organic compounds identified in studies of process air streams in certain municipal waste water treatment plants include hydrocarbons of all common sorts; aromatics including benzene, alkyl benzenes, toluene, xylenes, naphthalene and the like; oxygenated compounds such as alcohols, ketones and epoxides; halogenated compounds including chloroform, trichloroethylene methylene chloride and freons; nitrogenous compounds such as pyridine and various nitriles; and sulfur containing compounds including dimethyl disulfide and hydrogen sulfide. The concentration of individual compounds typically ranges in exhaust gases from a few parts per billion to a few hundred parts per million. Because of these low contaminant concentrations and because of the very large volume of air involved, ordinary treatment methods such as direct or catalytic combustion, absorption, adsorption, and the like, are either not applicable or are prohibitively expensive. Conventional technologies using recirculating chemicals first concentrate and then re-evaporate contaminants which recontaminates the exhaust gases.
2. Description of the Prior Art
One treatment method proposed in the patent literature for the removal of organic pollutants from air is that set out in the Merrill patent, U.S. Pat. No. 3,593,496. Merrill discloses that organic pollutants such as hydrocarbons can be removed from air by mixing the air with an aerosol of water droplets containing a surfactant which presents an oleophilic surface on the water droplets. The aerosol droplets absorb organic pollutants into and on their oleophilic surfaces and removal of the droplets from the air stream leaves a substantially purified air stream. Merrill prefers to form his droplets from aqueous suspensions of lecithin compounds as the surfactant.
Another process, which has come to be known as mist scrubbing, has recently been developed for removing contaminants, particularly odorous contaminants, from gas streams. This process uses an aqueous solution of one or more chemicals which are reactive toward the odorous contaminants. Contact between the reagent solution and the gas is accomplished by atomizing the aqueous chemical solution into very tiny liquid droplets and dispersing the droplets into the gas stream. The liquid droplets are sized such that they do not immediately settle out but instead drift with the gas much in the manner of a natural fog. Typical installations utilize droplets having a number median diameter on the order of about ten microns. Mist scrubbing processes are illustrated by U.S. Pat. Nos. 4,125,589 and 4,238,461.
In typical mist scrubbing processes, a suspension of atomized reagent droplets in an air stream is passed in concurrent fashion through a reaction chamber or scrubber vessel. It is usual practice to introduce the reagent droplet suspension into the top of the scrubber vessel and to remove a cleaned gas stream from the bottom of the vessel. The reaction vessel contains no packing or internal media of any kind and is sized to provide the desired reaction time, typically ranging from about five to thirty seconds, between the gas and reagent droplets.
Drain means are ordinarily provided at the bottom of the vessel to remove that spray liquid which settles out in the contacting step and the collected liquid is discharged as a waste stream. Depending upon reaction conditions, particularly contact time and the size distribution of the spray droplets, the amount of spray liquid which settles out, and is removed from, the vessel is normally less than the amount of liquid introduced into the scrubber vessel in the droplet spray. The remainder of the spray liquid is carried from the reaction chamber with the exiting cleaned gas stream either as a vapor or as a suspension of tiny droplets or is volatilized to saturated the gas stream. A nearly complete and stoichiometric reaction between the reagent and the gas contaminants can routinely be achieved. Because the reagents ordinarily used in odor removal are reduced to low concentrations and comprise chemicals such as sodium hypochlorite, sodium hydroxide and sulfuric acid, the escape of exhausted reagent droplets in the cleaned air stream is of no significant concern.
A study totally unrelated to gas scrubbing technology and concerning the concentrations of pesticides found in morning mists above agricultural fields suggests that process mechanisms analogous to those employed by Merrill may also occur in the natural environment. Researchers D. E. Glotfelty et al, writing in Nature, Volume 325, Pages 602-605, Feb. 12, 1987, reported that certain natural fogs contained unexpectedly high concentrations of pesticides, herbicides and other chemicals. Fog sampled in Beltsville, Md. and in the San Joaquin Valley of California was found to contain concentrations of some toxic substances that was many times higher than was predicted by calculation using Henry's Law. Concentrations of insecticides such as malathion and herbicides such as alachlor in the fog droplets was far higher than was the level of these compounds in the surrounding air.
Their reported data showed that enrichment into the fog droplets was more pronounced for hydrophobic pesticides than for hydrophilic ones. The authors proposed two hypotheses to explain the enrichment. One hypothesis was that the fog droplets contained solutes such as dissolved or colloidal organic material which increased the solubility of hydrophobic compounds thereby shifting the equilibrium to the solution phase. A second hypothesis was based upon the authors' observation that surface-active, non-pesticidal organic matter was present in the fog liquid as shown by its foamy, soapy appearance. Although the authors cautioned that they had no experimental verification, they considered it to be a reasonable conjecture that surface-active material might have been present in sufficient amounts to produce an organic film on the surface of the fog droplets. Thus, the surface-active organic matter presumed to be present at the air-water interface acted to enhance the uptake of pesticides into the aqueous phase in a manner reminiscent of the process described by Merrill.